The electrostatic voltmeter (ESV) which includes the present invention is a device capable of measuring electrostatic fields or potential without current flow through the device. Generally, these devices include a probe or sensor assembly working in conjunction with an associated voltmeter assembly which receives the signals from the probe and produces an output signal. Subsequently, the output signal may be used to drive an indicator, or to control an electrostatic process as a function of the measured electrostatic potential. Thus, the features of the present invention may be used in the printing arts and, more particularly, in an electroreprographic system to control a xerographic process. These electrostatic voltmeters are particularly well suited for measuring photoreceptor surface charge, which in turn allows for the automated adjustment of machine characteristics to achieve high quality reprographic output.
Heretofore, it has been established that a sensing electrode must be modulated with respect to the field being measured in order to accurately measure the field. Essentially, two methods of achieving the required modulation of the electrode are known. The first method requires that the electrode be stationary and that a vibrating element, or vane, be moved between a viewing port and the electrode itself to modulate the field which reaches the electrode. The second method utilizes a moving electrode which is vibrated relative to the surface being measured. Two embodiments of this method are commonly used. A first embodiment moves the electrode in a direction perpendicular to the surface being measured thereby directly varying the capacitance between the electrode and the surface. The second embodiment utilizes an aperture, placed between the electrode and the surface under test, to periodically capacitively couple the electrode and the surface through the aperture as the electrode is moved parallel to the surface. This may be done by affixing the electrode to the end of a vibrating device such as the tine of a tuning fork. An advantage of the second embodiment is that if the electrode is generally centered on the aperture, the motion of the electrode will decouple it from the surface twice during every vibration cycle, effectively doubling the frequency of the modulation as compared to the frequency of the vibrating system. Generally, both the first and the second embodiments utilize a tuning fork to reliably achieve the modulation of the electrode, either by a vane attached to one end of the fork, or by affixing the electrode directly to an end of the fork.
A number of approaches and designs have been proposed for electrostatic voltmeters, of which the following may be relevant:
U.S. Pat. No. 4,763,078 to Williams, issued Aug. 9, 1988, relates to a sensor for an electrostatic voltmeter which consists of a vibratory element supported on one end in the manner of a cantilever beam, a sensitive electrode on the vibratory element for measuring the potential, a driver for vibrating the vibratory element in a direction to vary the capacitive coupling between the electrode and the electrical field being measured, and an amplifier mounted directly on the vibratory element so as to be in synchronous motion with the electrode.
U.S. Pat. No. 4,720,682 to Ikushima et al., issued Jan. 19, 1988, discloses a surface electric potential sensor for detecting the potential on a surface in a non-contacting fashion. U.S. Pat. No. 4,625,176 to Champion et al., issued Nov. 25, 1986, describes a vibrating probe for measuring electrostatic potential associated with electrophotographic copiers and print machines. The probe is made from a single piezoceramic bender element surrounded by a shield at a known potential. U.S. Pat. No. 4,614,908 to Daniele et al., issued Sep. 30, 1986, relates to a probe for electrostatic voltmeters which measures the voltage on a photoconductive surface. U.S. Pat. No. 4,318,042 to Eda et al., issued Mar. 2, 1982, relates to an electrometer probe for measuring the electrostatic potential on the surface of a photoconductive drum, in an electrostatic machine. The probe includes an electrode which is in the form of a strip.
U.S. Pat. No. 4,149,119 to Buchheit, issued Apr. 10, 1979, teaches an electrostatic voltmeter having a probe sensor that is modulated using a rotating vane or shutter arrangement. The probe is also conditioned to receive both A.C. and D.C. signals which are amplified by a D.C. amplifier, where the A.C. signal from the probe is fed back to the D.C. amplifier to stabilize its output. U.S. Pat. No. 3,852,667 to Williams et al., issued Dec. 3, 1974, relates to a probe or sensor for an electrostatic voltmeter including a voltage sensitive electrode which is vibrated within a housing so as to vary the amount of the surface of the electrode which is directly exposed to an external electrical potential through an aperture in the housing.
A preferred approach devised to vibrate the electrode in an electrostatic field in order to measure the electrostatic surface potential, is illustrated in U.S. Pat. No. 5,212,451 to Werner, issued May 18, 1993, the electrostatic voltmeter employs a single balanced beam vibratory element, supported and balanced at its center and driven in a manner to cause a regular vibration of the beam. However, one difficulty with such an embodiment is maintaining electrical contact with an electrode or probe affixed to an end of the vibrating beam in a manner that not only minimizes damping of the beam's vibration, but also allows the continuous operation of the electrostatic voltmeter over repeated vibratory cycles.
U.S. Pat. No. 5,270,660 to Werner et al., issued Dec. 14, 1993, discloses an electrostatic voltmeter requiring a modulator that floats at a potential equal to the potential being measured; requiring a floating power supply to drive the modulator. The electrostatic voltmeter further described in Continuation application Ser. No. 08/099,290 by Werner (filed Jul. 29, 1993), previously incorporated by reference for its teachings, and the vibrating beam modulator disclosed by U.S. Pat. No. 5,212,451 enable an ESV having all signal processing accomplished at ground potential with only the electrode and shield "floating" at a different potential, thereby eliminating any need for a floating power supply. However, both of these embodiments place additional constraints on modulator construction and the materials that must be used to insure there is: (a) no stray charge in the probe that can be modulated; (b) no undesired leakage of the coupled signal to ground; and (c) no capacitive coupling between the electrode and the electromechanical mechanism used to drive the modulator. The present invention seeks to achieve these requirements by assuring that all electrically insulating material seen by the electrode is "antistatic" and that all direct coupling to ground is very low leakage with high breakdown voltage, and that the capacitance between the electrode and the beam driving mechanism be minimized.
In accordance with the present invention, there is provided an apparatus for measuring the magnitude of an electrostatic field, comprising: a balanced beam adapted to vibrate; an electrode operatively associated with said balanced beam and insulated therefrom, said electrode moving in unison with said balanced beam in the electrostatic field to produce a high voltage biased signal; a high voltage signal processing circuit; and a multi-stranded conductor for electrically connecting the shielded electrode to the high voltage signal processing circuit, said high voltage signal processing circuit generating, in response to the high voltage biased signal, a second signal, having a lower voltage than the high voltage biased signal, in proportion to the magnitude of the electrostatic field.
In accordance with another aspect of the present invention, there is provided an apparatus for producing a signal representative of the magnitude of an electrostatic field, comprising: a substrate having an electrical contact affixed to a top surface thereof; a vibratory element, mounted upon said substrate and having one end which is free to vibrate; an electrode operatively associated with the vibrating end of said vibratory element, said electrode adapted to move in unison with said vibratory element through the electrostatic field to generate the signal; and a multistranded conductor electrically connecting said electrode to the electrical conductor on the top surface of said substrate to enable the transfer of the signal from the electrode to the electrical contact.
In accordance with yet another aspect of the present invention, there is provided a high voltage conductor for electrically coupling an oscillating member, oscillating with respect to a second member, including a composite conductor including a plurality of carbon fiber strands, said composite conductor having a first end electrically connected to the oscillating member and a second end electrically connected to the second member.